WASHINGTON — When Russia shifted its military campaign to focus on eastern Ukraine this spring, senior officials in the Biden administration said the next four to six weeks of fighting would determine the war’s eventual path.
That time has passed, and officials say the picture is increasingly clear: Russia is likely to end up with more territory, they said, but neither side will gain full control of the region as a depleted Russian military faces an opponent armed with increasingly sophisticated weapons.
While Russia has seized territory in the easternmost region of Luhansk, its progress has been plodding. Meanwhile, the arrival of American long-range artillery systems, and Ukrainians trained on how to use them, should help Ukraine in the battles to come, said Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff.
“If they use it properly, practically, then they’re going to have very, very good effects on the battlefield,” General Milley told reporters traveling home with him this month after visiting Europe.
Pentagon officials said that meant Russia might not be able to make similar gains in neighboring Donetsk, which along with Luhansk forms the mineral-rich region of Donbas. Ukrainian troops have been battling Russian-backed separatists in Donbas since 2014, when Moscow annexed Crimea from Ukraine.
After weeks of bloody battles in the east — with as many as 200 Ukrainian soldiers killed daily, by the government’s own estimate, and a similar or higher toll among Russian troops, according to Western estimates — Russia holds roughly the same amount of territory in Donetsk as the separatists controlled in February before the invasion.
But U.S. officials say they expect Russia to soon take over the entire Luhansk region. One defense official said he anticipated that the twin cities of Sievierodonetsk and Lysychansk would fall in days, as Russian forces pounded the area with heavy artillery and “dumb bombs” — unguided munitions that inflict high casualties.
According to reports over the weekend, Russian forces had broken through the Ukrainian front line in Toshkivka, a town just outside Sievierodonetsk and Lysychansk. Seizing Toshkivka would put the Russians closer to being able to threaten Ukrainian supply lines to the two cities, the last major population centers in Luhansk that have not fallen to Russia. As of Monday, it was unclear which side held Toshkivka.
Russian ground troops have advanced slowly, in some cases taking weeks to move one or two miles, U.S. officials said. That might signal a lack of infantry soldiers or extra caution by Moscow after it experienced supply line problems in its disastrous first weeks of the war.
Several military analysts say Russia is at peak combat effectiveness in the east, as long-range artillery systems promised to Ukraine from NATO countries are still trickling in. Ukraine is hugely outgunned, they say, a stark fact that President Volodymyr Zelensky acknowledged last week.
“The price of this battle for us is very high,” he said in a nightly address. “It’s just scary. And we draw the attention of our partners on a daily basis to the fact that only a sufficient number of modern artillery for Ukraine will ensure our advantage and finally the end of Russian torture of the Ukrainian Donbas.”
President Biden on Wednesday announced an additional $1 billion in weapons and aid for Ukraine, in a package that includes more long-range artillery, anti-ship missile launchers, and rounds for howitzers and for the new American rocket system. Overall, the United States has committed about $5.6 billion in security assistance to Ukraine since Russia invaded on Feb. 24.
Mr. Zelensky and his aides have appealed to the West to supply more of the sophisticated armaments it has already sent. They have questioned their allies’ commitment to the Ukrainian cause and insisted that nothing else can stop Russia’s advance, which even by conservative estimates has claimed the lives of tens of thousands of civilians and soldiers.
Defense Secretary Lloyd J. Austin III urged Western allies last week to redouble their military aid to Ukraine, warning that the country “is facing a pivotal moment on the battlefield” in its nearly four-month fight with Russia. Mr. Austin and General Milley met with U.S. allies in Brussels to discuss how to further help Ukraine.
Pentagon officials expect that the arrival of more long-range artillery systems will change the battlefield in Donetsk, if not in Luhansk.
Frederick B. Hodges, a former top U.S. Army commander in Europe who is now with the Center for European Policy Analysis, said the war would probably last many more months. But he predicted that Ukraine’s forces — bolstered by heavy artillery from the West — would slow Russia’s advance and begin to roll back its gains by late summer.
“War is a test of will, and the Ukrainians have superior will,” General Hodges said. “I see the Ukrainian logistical situation getting better each week while the Russian logistical situation will slowly degrade. They have no allies or friends.”
Russia’s military is built for short, high-intensity campaigns defined by a heavy use of artillery, military analysts said. It is not prepared for a sustained occupation, or the kind of grinding war of attrition underway in eastern Ukraine that requires swapping out battered ground forces.
“This is a critical period for both sides,” said Michael Kofman, the director of Russian studies at C.N.A., a research institute in Arlington, Va. “Probably in the next two months, both forces will be exhausted. Ukraine has a deficit of equipment and ammunition. Russia has already lost a lot of its combat power, and its force is not well suited for a sustained ground war of this scale and duration.”
Russia will try to continue making mile-by-mile territorial gains, and then will probably harden its front lines with mines and other defenses against a Ukrainian counterattack, which is expected after the long-range artillery systems arrive on the battlefield, analysts said.
In recent days, neither force has been able to achieve a major breakthrough in its opponent’s front lines.
Even though terrain could change hands, “neither side has the mass to exploit minor gains,” Christopher M. Dougherty, a former Army Ranger and a defense analyst at the Center for a New American Security, said in a Twitter post this month. “The war now likely becomes a test of endurance.”
As a result, several military analysts said, Moscow and Kyiv will both rush reinforcements to the front lines.
“The race to resupply will be critical for both sides,” Col. John B. Barranco of the Marine Corps, Col. Benjamin G. Johnson of the Army and Lt. Col. Tyson Wetzel of the Air Force wrote in an Atlantic Council analysis.
“To replace its losses, the Kremlin may need to resort to sending in thousands more conscripts,” the officers said, adding that Ukraine will need to maintain its logistics lines and move forward ground-based weapons, including long-range artillery and unmanned aerial systems.
Analysts and former U.S. commanders offered differing forecasts on how the war might change.
Weaknesses in the Ukrainian military’s position are beginning to show — and are sowing concern. While some independent analysts have predicted that the Russian advance will be halted in Sievierodonetsk, U.S. government experts are not so sure. Some say they believe that the grinding Russian advance could continue and that the Russians could soon make more progress in areas where Ukrainian counterattacks have been successful.
The tactics Russia is using, according to current and former officials, are having a devastating effect in eastern Ukraine, wreaking so much destruction that Mr. Zelensky has said troops are fighting over “dead cities” where most civilians have fled.
Other analysts predict a back and forth that could stretch for months or even years.
“This is likely to keep going, with each side trading territory on the margins,” Mr. Kofman said. “It’s going to be a dynamic situation. There are unlikely to be significant collapses or major surrenders.”
Military and intelligence officials said Russia had continued to suffer severe losses and was struggling to recruit soldiers to refill its ranks. Morale is low in the Russian military, and problems with poorly maintained equipment persist, U.S. officials and analysts say.
The fight in the Donbas has become a deadly artillery duel that is inflicting heavy casualties on both sides.
Commercial satellite imagery of craters in eastern Ukraine suggests that Russian artillery shells are often exploding on the ground near Ukrainian trenches, not in the air above them. Airburst artillery kills soldiers in trenches more effectively.
Stephen Biddle, a military expert and professor of international relations at Columbia University, said the imagery suggested that the Russians were using old ammunition that had been poorly maintained.
But inefficient artillery can still be very destructive when employed en masse.
“Quantity has a quality all its own,” Dr. Biddle said. “If I were one of the infantry getting pounded in those trenches, I’m not sure how much better I’d feel knowing that Russian artillery could be even more lethal if it were better maintained and employed.”